1. Field of the Invention
The present invention relates to the field of injection mold methodology and apparatus for the manufacture of dental appliances such as dentures, retainers, prostheses and like dental appliances.
2. Description of the Related Art
Since the 1800s, dental appliances, such as full or partial dentures (comprised of a denture body or gum that held anchored artificial teeth), have been constructed utilizing a dental flask (mold container) which held a mold (also known as an investment) that was used to form the dental appliance. The mold is usually made from dental plaster or other suitable material and is molded around a wax model or “wax-up” of the desired dental appliance. The wax-up may include non-wax items such artificial teeth or wires in case of a retainer. The mold containing the wax-up is secured in the dental flask.
Once all the mold material in the dental flask has hardened around the wax-up, the wax-up is removed from the mold by heating the dental flask/mold. The removal of the wax-up leaves behind a cavity in the mold used to form the dental appliance. Artificial teeth, wire and the like, however, are not removed in this process and are retained into place in the mold/dental flask.
In the traditional method of forming the dental appliance, the dental flask is dissembled, and the mold opened to form two halves. Dental appliance material (such a semi-liquid or paste-like acrylic, polyurethane, or epoxy resins) is placed into the cavity portion of one of the halves. The mold and dental flask are then reassembled and compressed by vice or clamp to force the dental appliance material into all the nooks and crannies of the cavity within the mold. During this compression, excess dental appliance material is squeezed out of the two mold halves.
Steps are then taken to solidify, cure or harden the dental appliance material remaining in the mold/dental flask. During this solidification, final curing or hardening of the dental appliance material, the dental flask/mold is usually kept compressed according to the dental appliance material manufacturer's specification. Once the solidification has occurred, the clamps or vices are removed, dental flask is disassembled, and the mold is opened to release the completed dental appliance.
This traditional methodology and apparatus had several drawbacks. First and foremost was the method of placing the dental appliance material into the mold. By compressing the dental appliance material in the above-described manner, the dental appliance material would move the false teeth out of alignment as the excess dental appliance material was squeezed out of the mold. This required money consuming labor and time to correct defaults caused by this effect. Further, the presence of additional dental appliance material can cause a variety of distortions to the dental appliance itself during the formation process. This is especially true when forming partial dentures. Further, because the dental appliance material expands and shrinks during the heating and cooling of the solidification process, unless the dental appliance material is keep under constant pressure in the mold, it can cause distortions of the desired dimensions of the dental appliance. This expansion and contraction could cause porosity in the solidified dental appliance material (i.e., undesired air bubbles forming in the denture base). This porosity not only weakens the base, but causes surface roughness which is irritating to the dental appliance wearer.
To counter effect or limit the occurrence of the porosity, the traditional method of forming dental appliances relied on two clamps. The first clamp was used to make the initial compression on the dental flask/mold in squeezing the dental appliance material into place. This first clamp was then released and replaced with a specialized second clamp with spring resistance to hold the dental flask/mold together under resilient pressure. This second clamp held the dental flask/mold and were placed a container of hot water to heat the dental flask/mold for the purpose of solidifying the dental appliance material. This methodology of placing and then releasing pressure on the whole dental flask/mold potentially caused the misalignment of the dental flask/mold which results in faults in the formed dental appliance.
To overcome these and other problems of the traditional compression mold techniques for manufacturing dental appliances, there has been interest in the field toward the application of injection molding technology to the manufacture of dental appliances.
One such application is taught by U.S. Pat. No. 5,324,186 issued to Bakanowski issued on Jun. 28, 1994. This patent taught the use of a two-piece dental flask whereby the two-piece dental flask with four corners would have a bolt passing through a threaded aperture in each corner. This improvement would allow the dental flask to be tightened and held together under pressure in proper alignment. The dental flask halves also featured buttons and matching depressions to further aid in correct positioning and alignment of the mold. Each dental flask also featured vent channels for the release of any air trapped in the mold. The channels would also act as indicator when the mold was properly filled with dental appliance material (i.e., the dental appliance material would squirt out of the dental flask from the channels leading from the interior space of the mold when the mold was properly filled with dental appliance material).
For attachment of the injection equipment, the Bakanowski '186 dental flask featured threaded apertures located both on the top of each mold half and well as on the sides. The side aperture allows the connection of the injection equipment to the dental flask/mold for the forced injection of the dental appliance material into the mold.
The top threaded apertures are used for injecting mold or investment material (e.g., dental plaster) into the dental flask once the dental flask halves are assembled after filling the lower dental flask half with dental plaster. The top threaded apertures are also used to attach injection equipment so as to eject or remove the dental plaster mold from the dental flask after the two dental flasks halves have been taken apart to release the formed dental appliance.
The injection equipment is comprised of a pneumatically powered injector that is loaded with a cartridge of the dental appliance material that allows injection of dental appliance material without contamination by the operator.
There are limitations imposed on the Bakanowski '186 invention. First, the use of a two piece mold requires the cumbersome method injection of the dental mold material (dental plaster) to fill the second or top half of the flask and complete mold formation.
This injection methodology still leads to the formation of air pockets and resultant irregular surfaces on the mold impression of the dental appliance. The use of threaded apertures for injection, releasing, pressurizing and holding of the molds or dental flasks, requires the mold itself to be comprised of a certain hardness and strength to avoid stripping of the threads while under pressure. This strength and hardness requirement can lead to the limitation of materials that can be used and possibly increase construction costs for the dental flasks.
Finally, the use of a special pneumatic injection device can increase the cost of the overall system and lessen its universality.
The U.S. Pat. No. 6,187,761 issued to Petkow et al. on Feb. 13, 2001, is also an injection technology for the manufacture of dental appliances such as dentures. The Petkow '761 teaches the use of a three-piece mold, essentially a bottom dental flask portion, a top dental flask portion and a removable top cover for the top dental flask portion. This three piece dental flask avoids the problem of Bakanowski '186 invention which has to inject the dental plaster into the mold to fill the top portion or half of the dental flask. By utilizing a three piece mold, Petkow '761 can just paint on and pour in the dental plaster directly into the dental flask to form the top half of the mold.
The Petkow '761 invention further teaches the use of a compression rather than pneumatic injection of acrylic material into the mold. The Petkow '761 injection device is comprised of a cylinder with an aperture aligned with an injection aperture of the mold. The dental appliance material is then placed into the cylinder with a piston being placed over the acrylic. A locking cap and locking ring are then placed over the cylinder/piston. The mold and injector are then placed into a press which pushes down on the locking cap that causes piston to inject the acrylic into the mold. The locking ring can then engage the cylinder so as to fix the locking cap into position and thus secure the piston/dental appliance material under pressure. The pressure of the press is let off and the pressurized mold/compression device is removed for further curing of the dental appliance material in the dental flask.
The Petkow '761 invention accordingly requires a specialized pressure machine to apply the pressure to the piston to cause injection of the acrylic into the mold Another issue not addressed by the prior art is that one of the most widely used of the dental appliance materials is acrylic resin which is discolored (turns from pink to an undesirable black color, a sign of gum disease) when it comes into contact with metal, such as those found in various injection equipment.
Therefore, what is needed is an injection mold methodology and apparatus that does not utilize complicated machinery; that allows the dental flasks to be made of a variety of materials; that prevents the discoloration of acrylic resin based dental appliance materials; reduces the amount of error that can occur during dental appliance formation and enhances the accuracy of the formed dental appliance.